This invention relates to controlled cessation of a particular cell function.
One instance in which controlled cessation of a cell function is useful involves microbial fermentation of a desired product. To achieve a desired microbial cell population, the cells are allowed to undergo an exponential ("log") phase of cell growth. When a suitable cell population density is reached, it may be advantageous to halt cell-growth so that cellular resources and metabolic energy are not diverted from the production of the desired compound. Ideally, a mechanism to achieve this goal should be absolute, so that some cells in the population will not circumvent the growth control and overgrow the culture.
One way to control cell growth is by limiting the availability of one or more nutrients that the cell requires for growth. Once the limited amount of a nutrient is used, cell growth ceases. Cells may be mutated or engineered so that they require a nutrient in order to grow. For example, if the nutrient is a particular amino acid such as tyrosine, cells are mutated or engineered to create a defect in the cell's natural pathway for synthesizing that amino acid. The cells then may be provided the amino acid in an amount selected to allow them to achieve a desired cell population; when the cells reach that population, they become starved for the amino acid and cannot grow.